1. Field of the Invention
The present invention relates to an optical information recording medium, particularly, relates to an optical disc, wherein recording or erasing information is conducted by changing configuration of atoms constituting a recording layer of the optical information recording medium by irradiating light.
2. Description of the Related Art
A so-called phase-change type optical information recording medium has been well known as one of optical memory mediums, which is enabled to record, reproduce or erase information by irradiating a laser beam. Such a phase-change type optical information recording medium uses transition between crystal and amorphous or transition between two crystalline phases of crystal 1 and crystal 2.
A thin film of chalcogen system alloy is commonly used as a material for a recording layer of a phase-change type optical information recording medium. A thin film of GeSbTe system alloy or AgInSbTe system alloy among the chalcogen system alloys has been practically applied for an optical information recording medium as a rewritable optical disc.
A recording principle of such a rewritable optical disc is as follows. A recording layer immediately after being filmed is in an amorphous state and low in reflectivity. Therefore, the recording layer of the optical disc is heated by irradiating a laser beam first, and then whole area of the recording layer is conducted to be a crystalline state that is high in reflectivity. In other words, the optical disc is initialized. Generally, this initialization is conducted by irradiating a laser beam converged with a diameter of some μm to 100 μm on the optical disc that is rotating.
Irradiating a laser beam on the initialized optical disc locally melts a part of the recording layer. Then the optical disc is cooled down rapidly. A phase of the part of the recording layer results in changing into an amorphous state. Accordingly, optical characteristics such as reflectivity, transmissivity, and birefringence index of the recording layer change in response to the change of phase, and resulting in recording information.
Reproduction is conducted by detecting reflectivity difference between crystal and amorphous or phase difference between them by irradiating a weaker laser beam than that for recording.
Further, rewriting is conducted by overwriting on a pre-recorded record mark without processing through an erasing process by injecting recording peak power, which is superimposed on erasing power in low energy that causes crystallization, into the recording layer.
A laser device having a relatively high output is essential for initialization. In such a laser device having a high output, a beam diameter is made finer, so that beam density increases even in lower laser power. However, until now, it takes extremely long time to initialize an optical disc by scanning with a laser beam having a beam diameter of some μm to 100 μm.
Accordingly, a GeSbTe system material has been developed which enables initialization by lower laser power and is made by combining GeTe and Ge2Sb3 that are not a eutectic system. Then an AgInSbTe system material has been developed after the development of the GeSbTe system material. Initializing the AgInSbTe system material requires stronger laser power than laser power for initializing GeSbTe system materials.
A laser beam has been advanced to have a shorter wavelength and higher output. Consequently, an initializing apparatus installed with a laser device in high power has been introduced since the above-mentioned GeSbTe system and AgInSbTe system materials were developed. In this connection, a GeSbTe system material of eutectic system, which was hardly initialized in the past, has now been developed.
In the case of the GeSbTe system materials except for the materials practically applied, it is commonly known that a eutectic composition of Sb and Te transforms its crystalline state to amorphous state or vice versa.
The Japanese Patent Application Laid-open Publication Nos. 1-115685/1989, 1-251342/1989, and 1-303643/1989 disclose the range of composition such as adding a third element, particularly, adding Ge into a eutectic composition of Sb and Te that is composed of 70 atomic percent of Sb and 30 atomic percent of Te.
Further, the Japanese Patent Application Laid-open Publication No. 2000-313170 discloses that In (indium) is added to a eutectic composition for improving characteristics.
With respect to a recording method of an optical disc that is made from a phase change material, recording has been conducted by using a red laser beam having a wavelength of around 650 nm that has been used for recording DVD-ROM discs, or by using a laser beam having a wavelength being longer than 650 nm. Recently, however, a semiconductor laser element emitting light having a wavelength of around 400 nm has been introduced into the market.
Further, a numerical aperture (hereinafter referred to as NA) of an objective lens has been increased, and resulted in enabling to converge a light beam furthermore.
Combining a laser beam having a shorter wavelength and an objective lens having a higher NA makes a spot diameter of a laser beam smaller and results in increasing recording density of an optical disc. Consequently, an optical disc system using a blue laser beam has been studied.
Recently, specifications for an optical disc, a so-called Blu-ray Disc, which is recorded or reproduced by using a blue-violet laser beam having a wavelength of 405 nm and an objective lens having a NA of 0.85, have been formulated. Such a Blu-ray Disc draws public attention as next generation of optical discs.
In this connection, it is demanded for an optical disc enabling to realize high-density recording by using a blue laser beam, wherein a recording density conducted by a blue laser beam is higher than a recording density conducted by a conventional red laser beam, that the optical disc is not only capable of a blue laser beam having a shorter wavelength but also able to be recorded sufficiently even by a short pulse width, and further rewritable.
With respect to GeSbTe system materials in the neighborhood of eutectic composition out of conventional materials, it has been experimentally confirmed that the GeSbTe system materials enabled to be recorded and reproduced to some extent by using an optical disc system equipped with a blue laser beam.
Further, in order to improve recording and reproducing characteristics, it was also experimentally confirmed that controlling a crystallizing speed by adjusting balance of each amount of Sb and Te in compositional elements of the GeSbTe system materials could change a linear velocity corresponding to recording.
Furthermore, an amount of Ge significantly affects stability of recording materials. If the amount of Ge is within a moderate range, reproduction durability against a blue laser beam having a small beam diameter and a high energy density can be improved.
However, in order to perform recording in higher density, improving recording and reproducing characteristics furthermore is essential. In other words, both of increasing reproduction power and reducing jitter are essential to recording in higher density. On the contrary, in a case of an optical disc made from GeSbTe system materials as a recording material, there exists a limit in characteristics with respect to increasing reproduction power and reducing jitter.
By the constitutional compositions disclosed in the above-mentioned Japanese Patent Application Laid-open Publications, it is impossible to obtain such an optical information recording medium to be realized by the present invention that exhibits sufficient recording and reproducing characteristics and contrast and further higher durability against reproduction light.